In recent years, due to shortage of energy resources and the like, it has been required to improve the efficiency of a propulsion system configured to generate a propulsive force in a vessel. In the propulsion system of the vessel, a diesel engine has the most excellent heat efficiency among various prime movers, and the propulsion system in which the diesel engine is coupled directly or via a reducer to a propeller as a propulsor is now the mainstream. However, it has been pointed out that the diesel engine has an air pollution problem in terms of environmental performance. As an environmental countermeasure of the diesel engine, an electric propulsion system configured to rotate the propeller by an electric motor to generate the propulsive force has been attracting attention. For example, U.S. Pat. No. 6,692,319 discloses a ring-shaped propulsion device for submarine vessels, the propulsion device being configured such that propeller blades projecting in a radially inward direction are provided on a rotor of a ring-shaped electric motor. According to this propulsion device, by the rotation of the propeller blades driven by the electric motor, water stream is ejected to produce the propulsive force.